It is known to combine electrodynamic braking with friction braking in a mine hoist. In the combined scheme, the electrodynamic brake provides the major portion of the braking effort at higher hoist speeds and the friction brake provides the major portion of the braking effort at the lower speeds. The friction brake usually comprises a brake pad that is applied against a surface of the mine hoist. The brake pad is connected to a brake shoe that is in turn connected to a rod. The rod is connected to a piston housed within a chamber. Fluid such as air is forced into the chamber to pressurize same and act against the piston controlling the braking pressure applied by the brake pad against the hoist. As the brake pressure changes so does the deceleration of the hoist.
One such prior art combined braking system for a mine hoist is disclosed in Canadian Patent No. 922,009 issued Feb. 27, 1973 to Peter deh. Eastcott. The combined braking system has a regulator which regulates friction braking according to hoist deceleration. The friction brakes are applied to supplement the electrodynamic brake so as to control stopping of the hoist according to a prescribed deceleration pattern. This is achieved by comparing the actual deceleration of the mine hoist with a reference signal representing a hoist deceleration program to control the braking pressure applied by the friction brakes to hold the deceleration of the hoist to the program irrespective of the braking effort provided by the electrodynamic brake.
While Canadian Patent No. 922,009 teaches the use of combined regenerative electrodynamic braking and friction braking, it is current practise not to rely on the regenerative braking effect of the motor but to rely solely on the friction brake to decelerate the mine hoist in emergency situations. The aforementioned patent is silent on the type of friction brake used, however, the teachings of the aforementioned patent have been implemented by the assignee of the aforementioned patent in respect of air brakes. The air brakes included a pinch valve which closed after a predetermined amount of air exited the chamber to maintain the braking pressure. The air remained in the chamber providing a static pressure until such time as a pressure change in brake application was required. In the event more braking was required, the pinch valve opened and additional air escaped out of the chamber until the required pressure was obtained. At this time the pinch valve was closed. Such an air brake and pinch valve were well adapted to reducing braking pressure applied by the brakes to the hoist at a rate according to a deceleration program for the hoist as taught in the aforementioned Canadian Patent No. 922,009.
In the last few years, hydraulic brakes have become the preferred choice of friction brake used in mine hoist applications. This is due to the economic considerations. However, the preferred control for a hydraulic friction brake requires continual flow of the fluid, such as oil for example, through the brake. Brake pressure in the braking chamber is a result of a pressure drop across an outlet orifice of the chamber which orifice continually remains open to maintain braking pressure. In view of this, reducing braking pressure applied by a hydraulic brake to the hoist at a rate according to a deceleration program for the hoist in the manner taught by the aforementioned Canadian Patent No. 922,009 heretofore has not been achieved since the principle taught in this patent revolves around the use of a friction brake where the fluid in the braking chamber is air and remains at a static pressure.